A vane-type variable valve timing adjusting mechanism is, as shown in JP2001-159330A (U.S. Pat. No. 6,330,870B1), adapted in such a manner that a housing rotating in a timed relation to a crank shaft of an engine is disposed coaxially with a vane rotor connected to a cam shaft of an intake valve (or exhaust valve) and a plurality of vane-accommodating chambers formed in the housing respectively are divided into an advance hydraulic chamber and a retard hydraulic chamber by vanes (blade portions) at the outer periphery of the vane rotor. In addition, the hydraulic pressure in each hydraulic chamber is designed to be controlled by a hydraulic control valve to rotate the vane rotor relative to the housing, so that a displacement angle of the cam shaft (cam shaft phase) to the crankshaft is varied to variably control valve timing.
In the vane-type variable valve timing adjusting mechanism (hereinafter referred to as “VCT”), typically, the VCT shows a non-linear response characteristic to the electric current applied to a hydraulic control valve (hereinafter referred to as “OCV current”), in which a region where the response becomes slow exists around a point at which a holding current is applied for holding the VCT on a certain position. In this region, even if the OCV current is feedback-controlled in accordance with the deviation between the target displacement angle and the actual displacement angle, the movement of the VCT is still slow and the VCT cannot promptly respond or be driven in the direction of the target displacement angle.
When the feedback gain is excessively increased as a countermeasure, the overshooting occurs to deteriorate a convergent characteristic of an actual displacement angle to a target displacement angle, thereby producing the problem of deteriorating combustion of an engine or the like.